Y. M. Lu

Characterization of biaxial stress and its effect on optical properties of ZnO thin films

Biaxial stress of ZnO film deposited on quartz was measured by side-inclination x-ray diffraction technique, indicating that the film is subjected to a tensile stress. One part of the stress is induced by thermal mismatch between the ZnO and the quartz and increases with annealing temperature, while another part results from lattice mismatch and is about 1.03GPa. The optical band gap of the ZnO film shows a blueshift with increasing biaxial tensile stress, opposed to the change of the band gap with biaxial tensile stress for GaN. The mechanism of the stress-dependent band gap is suggested in the present work.

Nitrogen-related recombination mechanisms in p-type ZnO films grown by plasma-assisted molecular beam epitaxy

The recombination mechanisms of nitrogen-related emissions in p-type ZnO films were investigated by photoluminescence (PL) measurements. The enhanced peak at 3.260 eV was confirmed as a donor-acceptor pair (DAP) transition and the emission around 3.310 eV was assigned to the free electron to acceptor (FA) recombination by studying the temperature evolution of DAP and FA luminescence, calculating the energy level of the corresponding nitrogen acceptor, and measuring the decay time of DAP transition. By comparing the PL spectra of the samples with various hole concentrations, it can be found that as the hole concentration increases, the DAP peak significantly dominates the spectra. In addition, the acceptor binding energy is estimated to be about 120 meV from the FA transition, which is in good agreement with the value calculated by a hydrogenic acceptor model.

Formation of p-type MgZnO by nitrogen doping

A wurtzite N-doped MgZnO film with 20at.% Mg (MgZnO:N) was grown by plasma-assisted molecular beam epitaxy on c-plane sapphire using radical NO as oxygen source and nitrogen dopant. The as-grown MgZnO:N film behaves n-type conduction at room temperature, but transforms into p-type conduction after annealed for 1h at 600°C in an O2 flow. The p-type MgZnO:N has a hole concentration of 6.1×1017cm−3 and a mobility of 6.42cm2∕Vs. X-ray photoelectron spectroscopy measurement indicates that substitution of N for O site is in forms of N atom (N)O and N molecule (N2)O for the as-grown MgZnO:N, but almost only in a form of (N)O for the annealed MgZnO:N. The mechanism of the conduction-type transition induced by annealing is discussed in the present work.

Resonant Raman scattering and photoluminescence from high-quality nanocrystalline ZnO thin films prepared by thermal oxidation of ZnS thin films

In this paper, we report photoluminescence (PL) from high-quality nanocrystalline ZnO thin films. The high-quality nanocrystalline ZnO thin films are prepared by thermal oxidation of ZnS films deposited by low-pressure metal organic chemical vapour deposition technique. X-ray diffraction indicates nanocrystalline ZnO thin films with a polycrystalline hexagonal wurtzite structure. The Raman spectrum shows a typical resonant multi-phonon process within the ZnO film. A strong ultraviolet emission peak at 380 nm is observed and the deep-level emission band is barely observable at room temperature. The strength (ΓLO) of the exciton-longitudinal-optical (LO)-phonon coupling is deduced from the temperature dependence of the full width at half maximum of the fundamental excitonic peak. ΓLO is largely reduced due to the quantum confinement effect. The origin of the luminescence is discussed with the help of PL spectra.

Biaxial stress-dependent optical band gap, crystalline, and electronic structure in wurtzite ZnO: Experimental and ab initio study

The relationship between band gap and biaxial stress in wurtzite ZnO thin films has been investigated by side-inclination x-ray diffraction technique and optical absorbance spectrum as well as ab initio calculation. The experimental result shows that differing from other semiconductor thin films with hexagonal structure, such as GaN, the band gap of ZnO thin films increases with the increase in biaxial tensile stress. For explaining the difference, ab initio calculation is performed to simulate the relationship between band gap and biaxial stress of wurtzite ZnO and GaN. The calculated result indicates that the band gap of ZnO increases under biaxial tensile stress but GaN is opposite, supporting our experimental result. The band offset calculation shows that the conduction-band minimum (CBM) and the valence-band maximum (VBM) of ZnO and GaN offset to low energy under biaxial tensile stress. The VBM offset of ZnO is larger than the CBM, responsible for the increase in band gap. The VBM offset of GaN is sm...

Valence-band offset of epitaxial ZnO∕MgO (111) heterojunction determined by x-ray photoelectron spectroscopy

The valence-band offset of ZnO∕MgO (111) heterojunction has been directly measured by x-ray photoelectron spectroscopy. Excluding the strain effect, the valence-band offset is determined to be 0.87±0.20eV, and the conduction-band offset ΔEC is deduced to be −3.59±0.20eV, indicating that ZnO∕MgO heterojunction has a type-I band alignment. The conduction-band and valence-band offset of MgO∕ZnO is used to interpret the origination of p-type conduction in undoped MgxZn1−xO alloy and deeper acceptor level in undoped and N-doped p-type MgxZn1−xO alloy than in ZnO.

Realization of p-type conduction in undoped MgxZn1−xO thin films by controlling Mg content

Undoped MgxZn1−xO thin films with Mg content of 0⩽x⩽0.20 were grown on c-sapphire substrate by plasma-assisted molecular beam epitaxy. The MgxZn1−xO shows n-type conduction in Mg content of x⩽0.05, and the carrier concentration decreases slowly from 1018to1017cm−3 with increasing Mg content. However, as x⩾0.10, the MgxZn1−xO begins to show p-type conduction, and the carrier concentration goes down sharply to 1015cm−3 firstly and then increases slowly with increasing Mg content from 1015to1016cm−3. The mechanism of transformation from n to p type and change of the carrier concentrations with Mg content were investigated by photoluminescence and absorption measurements as well as first-principle calculation.

A direct first principles study on the structure and electronic properties of BexZn1−xO

We present a systematic study on the structural and electronic properties of all alloy configurations of BexZn1−xO in a unit cell with 16 cations using density functional theory (DFT) methods. The 216 complexity is reduced by considering the symmetry of the parent structures. The experimental structures and electronic properties of the bulk material are reasonably reproduced by the DFT methods. The lattice constants of the alloy are found to follow Vegard’s law [Z. Phys. 5, 17 (1921)] and are comparable with the experimental values. Examining the formation enthalpy of all alloy configurations suggests the possible existence of three metastable order states. The calculated band gap of the BexZn1−xO is also compared with the experimental measurements and the authors found that some alloy configurations with the same concentration can have band gaps differed by ∼1.5eV.

Zn0.76Mg0.24O homojunction photodiode for ultraviolet detection

Zn0.76Mg0.24O p-n photodiode was fabricated on (000l) Al2O3 substrate by plasma-assisted molecular beam epitaxy. Ni∕Au and In metals deposited using vacuum evaporation were used as p-type and n-type contacts, respectively. Current-voltage measurements on the device showed weak rectifying behavior. The photodetectors exhibited a peak responsivity at around 325nm. The ultraviolet-visible rejection ratio (R325nm∕R400nm) of four orders of magnitude was obtained at 6V bias. The photodetector showed fast photoresponse with a rise time of 10ns and fall time of 150ns. In addition, the thermally limited detectivity was calculated as 1.8×1010cmHz1∕2∕W at 325nm, which corresponds to a noise equivalent power of 8.4×10−12W∕Hz1∕2 at room temperature.

Excitonic electroluminescence from ZnO-based heterojunction light emitting diodes

This work reports on the fabrication and characteristics of n-ZnO/p-GaN and n-ZnO/n-MgZnO/n-ZnO/p-GaN heterojunction light emitting diodes (LEDs). Both devices exhibited diode-like rectifying current-voltage characteristics. Room temperature electroluminescence (EL) spectra for both LEDs consisted of dominant emission at 375 nm and two weaker bands centred at 415 nm and 525 nm, which were attributed to ZnO excitonic transition and defect-related emissions from GaN and ZnO, respectively. Moreover, it was demonstrated that the single heterojunction required a higher injection current to obtain an excitonic EL than that for the n-ZnO/n-MgZnO/n-ZnO/p-GaN LEDs. This indicated that the insertion of the MgZnO layer confined the injection carriers and thus increased the intensity of excitonic emission in the ZnO active region.